Laser welding of synthetic resin materials can, for example, be conducted as described below. As shown in FIG. 1, one part incorporating a laser ray transmitting material and another part incorporating a laser ray absorbing material are brought into contact with each other. When irradiating laser ray from the laser ray transmitting material side to the laser ray absorbing material, the laser ray that has transmitted the laser ray transmitting material is absorbed in the laser ray absorbing material and generates heat. By this heat, the laser ray absorbing material is molten around the portion that has absorbed the laser ray, and the laser ray transmitting material is also molten, the resins of the two parts fuse together, and upon cooling sufficient welding strength is obtained and the laser ray transmitting material and the laser ray absorbing material are joined firmly. Features of laser welding include the capability of welding without bringing the laser ray generation portion in contact with the portion to be welded, the minimal thermal effect on the surrounding portion because the heating is localized, freedom from the problem of mechanical vibration, the capability of welding of fine portions and structures, high reproducibility, maintenance of high air-tightness, high welding strength, inconspicuous welded portion, and no generation of dust etc.
Traditionally, resin parts have been joined together by clamping with clamping parts (bolts, screws, clips, etc.), adhesion using adhesives, vibration welding, ultrasonic welding, etc. According to laser welding, labor saving, productivity improvements, production cost reductions, etc. can be achieved because secure welding is achieved by simple operation to provide strength equivalent to or more than the levels expected by conventional method and in addition vibration and heat have minimal effects. With these features, laser welding is suitable for the joining of functional components, electronic components, etc., for which the avoidance of the influence of vibration or heat is desired in, for example, automobile industry, electric/electronic industry and other fields, and is applicable to the joining of resin parts of complex shapes.
As a technology concerning laser welding, Japanese Patent Laid-Open No. HEI-11-170371 describes a method of laser welding comprising a process wherein laser ray is irradiated so that it focuses on a portion where an opaque part comprising a laser ray absorbing thermoplastic synthetic resin and a colorless transparent part comprising a laser ray transmitting thermoplastic synthetic resin are in contact with each other. In this case, however, when viewed from the colorless transparent part side, the welded portion differs from the non-welded portion in color and smoothness, posing a problem of poor appearance.
Additionally, WO02/36329 describes laser welding of a polyolefin resin using an anthraquinone dye and a perinone dye as colorants. However, if a common neutral dye is used to color a polyolefin resin, the dye is unlikely to fix due to the weak binding of polyolefin resins to dyes and in addition because the neutral dye used is very highly soluble in resins, posing a problem of the bleeding phenomenon, in which the dye migrates elsewhere in the event of contact with another polyolefin resin. This has been a critical problem not only in the use of laser welding, but also in general situations of coloring polyolefin resins.
The present invention has been developed in view of the aforementioned problems in the prior art, and is intended to provide a laser ray transmitting colored polyolefin resin composition that is highly transmittable for laser ray wavelength band (wavelengths from 800 nm to 1200 nm, e.g., 808, 820, 840, 940, and 1064 nm), that undergoes no color fading of the colored polyolefin resin part in the heat treatment process prior to laser welding of the resin part, that permits laser welding with substantially no sublimation of dye, and that possesses sufficient anti-bleeding quality, and a method of laser welding using the same laser ray transmitting colored polyolefin resin composition.